May i6, 1921 
Temperature Relations of Cladosporium citri 
251 
often becomes very severe in groves if the weather is cold and wet during the develop¬ 
ment of the first spring growth. 
Again: 
Some trees in scabby groves under observation during the past two years have 
retained the late starting habit and remained practically free from the disease. 
Again: 
During the early spring of 1915 the air was very moist and cold while the first growth 
was in its early stages and as a result sour scab developed in great abundance even 
on high sandy land if growth was early and vigorous. 
Stevens ( io ) says of citrus scab: 
If cool wet weather prevails at the time the new growth is putting out or at the time 
the fruit has set, the disease is apt to be severe ... in groves where scab has become 
established. 
In the light of these temperature experiments, the greater severity 
of scab at the low spring temperatures of Florida seems to be due largely 
to the fact that these temperatures fall mainly within the range for 
infection, while the later spring and summer temperatures are usually 
too high for infection to take place, even though other conditions are 
favorable. An examination of the temperature data for Tampa, Fla., 
given in Table IV suggests that this last statement may be translated 
into terms of mean temperatures, to the effect that severe infection will 
usually occur when the mean temperatures are well within the infection 
range and that infection will be unlikely to take place when the mean 
temperatures are outside this range. 
Table IV .—Mean temperatures at Tampa , Fla. 
[In degrees centigrade] 
Month. 
1909 
1910 
19H 
19x2 
1913 
1914 
1915 
X9l6 
1917 
1918 
February. 
16. 5 
i 5-5 
i 7-5 
I 3 - 5 
18. s 
i 5-5 
iS- 5 
16.5 
16.5 
20 
March. 
i 9 - 5 
19 
20 
20. 5 
21 
16.5 
14. 5 
17 
20. 5 
22 
April. 
23 
20. 5 
23 
23 
21 
22 
20. s 
20. 5 
22 
22 
Mean.... 
i 9 - 5 
18. S 
20 
19 
20 
18 
17 
18 
20 
21 
May. 
24- 5 
24. 5 
24- 5 
25-5 
24 
25 
26 
25 
24 
24-5 
June. 
27 
26. 5 
27 
26 
2(3 
27 
28 
26 
26. 5 
27 
July . 
26. 5 
26. 5 
27 
28 
27 
27 
28. 5 
28 
28 
28 
Mean. 
26 
26 
26 
26. 5 
25-5 
26. 5 
27 - S 
26. 5 
26 
26. 5 
The average of the monthly mean temperatures for February, March, 
and April for this io-year period was between 17 0 and 21 0 C., while the 
average of the monthly mean temperatures for May, June,, and July was 
between 25.5 0 and 26.5°. All the monthly mean temperatures for the 
first period were well within the infection range for scab, while all the 
